1. Field of the Invention
The present invention generally relates to mining and construction tools and, more particularly, is concerned with an improved butterfly-type shim having perforations through its mid-section and a double sandwich braze joint produced between a bit body and an insert tip which incorporates such shim.
2. Description of the Prior Art
Considering that thermal expansion rates of hard carbide and tungsten alloys vary from one-third to one-half that of steel, over the years brazing has proven to be an extremely successful and advantageous method of mounting carbides to steel. While it is most satisfactory when applied to small-area, short-length joints, it can also be satisfactorily applied to larger joints. However, whereas small joints are produced by relative simple, straightforward construction methods, large joints normally require more complicated methods to avoid difficulties which would otherwise be encountered in construction of the large joints.
These difficulties stem from the amount of thermal or brazing strain (or stress) created in the joined parts during formation of the braze joint. The amount of brazing strain created in small areas or short lengths of parts as a braze joint between them solidifies is usually minor and normally absorbed without materially weakening the joined parts. On longer parts or larger areas, however, the brazing strain becomes proportionately greater and can lead to considerable bending and frequently to fracture of the carbide part. Thus, more complicated methods of construction have been used heretofore to relieve brazing strains induced in carbide and steel parts by these longer or larger braze joints.
One method of construction to relieve brazing strain is to use a sandwich braze joint composed of a shim of a malleable metal, such as copper, between the carbide and steel parts. The malleable metal shim does not melt during brazing of the joint and will deform under the brazing strain without losing its bond to the steel or carbide parts, thus relieving the stress or strain to a large degree. Other methods of construction are to use a copper shim assembled between two shims of silver solder, or to use a three-ply shim composed of a layer of silver solder on either side of the copper.
One prior shim having a butterfly design has been proposed heretofore to form a double sandwich-type braze joint for mounting a carbide insert tip in an elongated groove defined in a bit body. This butterfly-type shim is generally U-shaped having upstanding side walls spaced apart and interconnected by a solid bottom wall. However, when the shim is placed in the groove between the interior bottom and opposite side surfaces of the bit body which define the groove and the exterior bottom and opposite side surfaces of the insert tip which is fitted into the groove, the respective exterior and interior surfaces on the side walls and bottom wall of the shim make flush contact with the adjacent bit body and insert tip surfaces.
Although this prior art butterfly-type shim having a solid bottom is intended to provide a double sandwich-type braze joint, the solid bottom of the shim substantially impedes the necessary distribution of molten brazing alloy across the bottom of the groove for setting up of capillary flow of the braze alloy upwardly between the adjacent surfaces of the bit body, insert tip and shim. Thus, the double sandwich joint cannot be produced effectively and consistently using this prior shim design. Formation of an incomplete and thus ineffective braze joint will limit the reliability and useful life of the tool.
Representative examples of prior art shims include those shown in U.S. Pat. Nos. (1,956,233) Braun, Orr et al (4,330,044) and Dziak (4,356,873); Swiss Patent to Braun (No. 227,635); German Patent Application to Hasewend (No. B 28706Ib/49h); and British Patents Nos. 386,909, 664,983 and 668,810. Several of these prior art shims have perforations formed through their respective wall portions and distributed uniformly across the wall portions between the opposite ends thereof. However, these shim constructions with uniformly distributed perforations, while reducing residual stresses in the joint, would result in a weakened braze joint subjecting the carbide insert tip to being broken off.
Consequently, a need exists for an improved technique to produce a more effective braze joint between the bit body and insert tip so as to provide a more durable tool having an increased useful life.